1. Field of the Invention
This invention relates to a fusing device, an image forming apparatus, and a belt.
2. Description of Related Art
With an image forming apparatus such as, e.g., a printer, a photocopier, a facsimile machine, or the like, for example, with an electrophotographic printer, a fusing device of a heating roller type has been conventionally disposed to fuse an toner image transferred onto a paper serving as a recording medium, and the fusing device has a fusing roller and a pressure roller, in which the toner image is fused onto the paper while the paper transits through a nipping portion formed between the fusing roller and the pressure roller.
On the other hand, with a fusing device of a belt type capable of fusing with use of an endless belt (hereinafter referred to as a “fusing belt”), electric power can be saved compared with the fusing device of the heating roller type, and necessary duration for changing a status of the fusing device from suspended into ready for fusing, i.e., duration of increasing temperature can be shortened (see, e.g., Japanese Patent Application Publication No. H6-348,156).
FIG. 2 is a cross-sectional view showing an essential part of the conventional fusing device of the belt type.
In FIG. 2, numeral 11 is a fusing belt disposed with a halogen lamp 21, rotated in a direction of arrow a, numeral 12 is a pressure roller disposed as in contact with the fusing belt 11, and numeral 13 is a pushing member disposed inside the fusing belt 11, pushing the fusing belt 11 onto the pressure roller 12, forming a nipping portion N between the fusing belt 11 and the pressure roller 12.
When the pressure roller 12 is rotated in a direction of arrow R upon driving motor M, the fusing belt 11 is driven to move with frictional force exerting between the pressure roller 12 and the fusing roller 11, in a direction of the arrow a while sliding on the pushing member 13. When the halogen lamp 21 renders the nipping portion N have a prescribed temperature and paper P is conveyed in a direction of arrow b at the nipping portion N, toner image T is fused onto the paper P.
In that case, since the fusing belt 11 does not need to be formed as stretched among a plurality of members such as, e.g., a tension roller or the like, thermal radiation of portions other than the nipping portion N can be reduced upon enhancing thermal insulation property and reducing thermal capacity of the pushing member 13. Therefore, the duration of increasing temperature can be shortened to improve the fusing device so as to start quickly.
It is to be noted that when conveyance speed of the paper P is accelerated to accelerate speed of the fusing device, the paper P needs to be supplied with sufficient heat. Thus, a width of the nipping portion N (hereinafter referred to as a “nipping width”) is widened to increase a pressure of the nipping portion N.
In the meantime, on each of outer circumferential surfaces of the fusing belt 11 and the pressure roller 12, a surface layer made of material having high mold releasing property such as, e.g., a fluoric resin such as, e.g., a perfluoroalkylvinylether copolymer resin (hereinafter referred to as a “PFA”), a polytetrafluoroethylene (hereinafter referred to as a “PTFE”), or the like is to be formed to prevent toners composing the toner image T, paper dusts, etc. from attaching in accordance with the conveyance of the paper P.
Although the conveyance speed of the paper P and the moving speed of the fusing belt 11 depend on rotating speed of the pressure roller 12, the rotating speed easily changes when the pressure roller 12 has large thermal expansion amount, so that the paper P is occasionally pulled by the fusing device when striding across a transfer unit and the nipping portion N in an image forming unit, and in this case, the toner image undesirably lengthens or shifts to deteriorate image quality.
In that case, where a thickness of an elastic layer composing the pressure roller 12 is reduced to reduce the thermal expansion amount, it becomes hard to obtain the sufficient nipping width, so that the fusing device can not be accelerated. Thus, the outer circumferential surface of the pressure roller 12 is covered with a tube made of the fluoric such as, e.g., the PFA or the like, and a stress is applied on the pressure roller 12 from the outside to reduce the thermal expansion amount of the pressure roller 12 so that the rotating speed is suppressed from changing.
However, with the above described conventional fusing device, when the outer circumferential surface of the pressure roller 12 is covered with the tube made of the fluoric resin such as, e.g., the PFA or the like, the frictional force between the pressure roller 12 and the fusing belt 11 as well as the frictional force between the pressure roller 12 and the paper P are reduced, so that depending on a paper type, change of circumstances, or the like, a case may occur where neither the fusing belt 11 can be smoothly moved, nor can the paper P be smoothly conveyed.
It is an object of this invention to solve the above problems and further to provide a fusing device, an image forming device, a belt capable of moving the belt to smoothly convey the recording medium.